Preparation of direct positives by development with leuco derivatives



nited States Patent 3,531,283 PREPARATION OF DIRECT POSITIVES BY DEVELOPMENT WITH LEUCO DERIVATIVES Anne-Marie Accary-Venet and Jacques Pouradier, Vincennes, France, assignors to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey No Drawing. Filed Apr. 1, 1965, Ser. No. 444,830 Claims priority, application France, Dec. 1, 1964,

Int. Cl. G03c 7/00 US. Cl. 96-54 Claims ABSTRACT OF THE DISCLOSURE This invention relates to photographic processes in which a direct positive image is produced and more particularly to photographic processes in which a direct positive image is produced by the development of a latent image in a silver halide emulsion with a leuco compound.

Leuco compounds have been used prior to this in vention to develop negative images in exposed silver halide emulsions. According to one such process a latent image is developed with a leuco compound which is oxidized and immobilized in the exposed areas. The unoxidized leuco compound in the unexposed areas is then transferred to an image-receiving element while no transfer of the oxidized compoundin exposed areas takes place. The disadvantage of such process, and of all processes in which a negative is produced, is that a multiplicity of steps are required in order to produce a negative first and then a positive.

An object of this invention is to produce a direct positive image in a silver halide emulsion using a leuco compound as a developer.

According to this invention a direct positive image is produced in an exposed silver halide emulsion using as the developer a leuco compound which is oxidizable, preferably in air, to a dye. The exposed emulsion is contacted with the leuco compound, which reduces a portion of the silver halide in exposed areas to metallic .silver. The leuco compound is absorbed unchanged by the unexposed areas of the emulsion. Development is stopped before the density in the exposed areas reaches 0.2 and the emulsion is then contacted with an aqueous hydrohalic acid solution containing dissolved oxygen which causes the leuco compound to be oxidized to the corresponding dye in the unexposed areas while the exposed areas remain light in color, the oxidizing agent being used for reoxidizing metal silver to silver halide, thereby producing a direct positive image in the silver halide emulsion. The print is heated to a temperature of about -80 C., for instance in a glazer, in order to intensify the image, then it is fixed.

Any of the usual photosensitive silver halide emulsions, such as emulsions of silver chloride or silver bromide in gelatin, may be used as the photosensitive emulsion in this invention. Emulsions of this type are well known in the art. The emulsion is spread out as a film on a suitable photographic paper which serves as a backing 3,531,283 Patented Sept. 29, 1970 sheet. Films having an emulsion speed suitable for contact printing are preferred in the present invention. Silver bromide is a particularly preferred silver halide.

The present process is especially suitable for the re production of material in the form of lines, i.e., printed matter, line drawings, charts, graphs, and the like. Such material on an ordinary printed page can be reproduced according to this invention. However, it is much more convenient, particularly for office copying machines, if the material is in the form of opaque lines or' characters on a light-transmitting support member. Preferably this light-transmitting member is a transparent film. A preferred mode of operation is to place a copy paper having a silver halide emulsion film in contact with a transparent film containing the material to be copied, and to expose the silver halide emulsion through the transparent film. Projection exposure may also be used.

The exposure times for the photographic film are the same as those which would be used if the film were to be developed wth a conventional photographic developer solution.

After exposure, the photographic film is developed in a developer which consists essentially of an aqueous solution of a leuco compound which is oxidizable by atmospheric oxygen to form a dark colored dye. The leuco compound corresponding to methylene blue is particularly suitable for this purpose. Solutions of the leuco derivative of methylene blue are either neutral or alkaline (pH 7 or greater) as this leuco compound is not oxidized in acid solution. Leuco compounds corresponding to other dark colored dyes may also be used. For instance, excellent results have also been obtained using the leuco derivative of crystal violet. Leuco compound corresponding to other dyes such as methyl violet and basic fuschin can also be used, although generally the positives obtained using these developers are not of as high quality as those in which the developer is a leuco compound corresponding to either methylene blue or crystal violet.

The leuco compounds of this invention are used in dilute concentrations. For example, excellent results have been obtained with the leuco derivative of methylene blue in a concentration of about 0.5 gram per liter. Good results have been obtained with the leuco derivative of crystal violet in a concentration of about 2 grams per liter. Appropriate concentrations of these and other leuco compounds which may be used in this invention can be determined experimentally by those skilled in the art.

The leuco compounds of this invention are not stable and therefore should be prepared at the time of use. These compounds may be prepared by reducing the corresponding dye in aqueous solution with an aqueous solution of sodium hydrosulfite in an amount just sufiicient to discharge all color in the original dye. It is important that no excess of sodium hydrosulfite be present in the developer solution. To insure that there will be no excess hydrosulfite present, a slight amount of dye, just suflicient to produce a lasting color, may be added to the solution after all of the original dye has been reduced by the sodium hydrosulfite. This quantity of sodium hydrosulfite is no greater than and preferably slightly less than the stoichiometric quantity for reduction of the dye to its leuco derivative. The leuco compounds are slowly oxidizable in air. Hence when a given developer solution is used for a substantial length of time, it may be necessary to add sodium hydrosulfite as color reappears.

The leuco compounds in the above indicated concentrations require only brief development times, generally in the order of about 10 seconds at room temperature (about 20 to 24 C.). A low density image is obtained upon development of the film with the leuco compound. The exposed areas are black, and contain small grains of 3 metallic silver formed by reduction of silver halide with the developer. Probably some dye is also present in these areas as a result of oxidation of some of the leuco compound. Leuco compound is absorbed by the silver halide emulsion in the unexposed areas.

After development, the print is immediately immersed in an aqueous solution of hydrochloric acid at room temperature (20 to 24 C.). A preferred concentration of hydrochloric acid is about 1 molar. Hydrobromic acid may be used instead of hydrochloric acid if desired, but generally with less uniform results. Atmospheric oxygen dissolved in the hydrochloric acid acts as an oxidizing agent which oxidizes the silver in the exposed areas, probably to silver chloride, and simultaneously oxidizes the leuco compound in the unexposed areas to the corresponding dye. The leuco compound in the exposed areas remains unchanged. In this manner a positive image, of less intensity than the image ultimately desired, is produced. It will be noted that this image is a direct positive image which does not require the prior formation of a negative image on a separate film.

Leuco compounds which are oxidizable in air are preferred so that atmospheric air can be used as the oxidizing agent. Of course, it is understood that oxidizing agents may be added to the hydrochloric acid solution if desired. Quinone is one example of such oxidizing agent. When an oxidizing agent is added, a leuco compound which is not readily oxidizable to the corresponding dye by atmospheric oxygen may be used.

In order to intensify the positive image, the temperature of the developed and printed photographic emulsion is raised. A drying temperature of about 70 to 80 C. is preferred, since within this temperature range the image is readily and reliably intensified to the desired degree of contrast. The image is then fixed by means of a fixing agent, preferably thiourea, so that no further development will take place.

It will be seen that the process of this invention produces a direct positive image in a silver halide emulsion using a minimum of steps which are easily carried out.

This invention will now be described further with reference to specific embodiments thereof, as illustrated in the following examples.

EXAMPLE 1 A sample of Kodak VG 5 Velox paper (a silver bromide photographic printing paper of speed suitable for contact printing) is exposed by placing the printing paper in contact with a transparent film containing opaque printed matter which is to be reproduced. The exposure time is such that a satisfactory print would be obtained if the paper were to be developed in a conventional photographic developer solution.

After exposure, the paper is developed in a developer prepared as follows: One liter of an aqueous solution of methylene blue containing 0.5 gram per liter of the dye is reduced with freshly prepared aqueous sodium hydrosulfite solution until the color of the dye has just been discharged indicating transformation of the dye to its leuco derivative. About 4.5 cc. of sodium hydrosulfite is required. Then a sufiicient amount (about cc.) of an 0.5% solution of methylene blue is added until a lasting color is obtained. This lasting color probably indicates the destruction of any excess hydrosulfite which may have been added.

The exposed paper is soaked for 10 seconds in the developer prepared in the preceding paragraph. This de veloper is maintained at 20 to 24 C. A low density image is obtained in which the exposed areas contain a small amount of finely divided black metallic silver dispersed in the silver halide emulsion. The unexposed areas contain unreduced silver halide emulsion which has absorbed unreacted leuco compound.

After development, the print is immediately immersed in a solution of 1 molar hydrochloric acid containing dissolved atmospheric oxygen maintained at 20 to 24 C. This destroys the metallic silver image without oxidizing the leuco compound retained in the exposed areas. However, in the unexposed areas, Where no metallic silver is present, the atmospheric oxygen contained in the hydrochloric solution oxidizes the leuco compound so that these areas turn blue. In this way a blue positive image of low density is obtained. The print is then drained and dried on a glazer between sheets of filter paper. As a result of such drying, densities are considerably intensified. The drying temperature is to C. All of the foregoing steps are carried out in inactinic light.

The densities of the positive image are correct at this stage, but the image is not stable to light as it still conains silver halide of the initial emulsion plus leuco derivative in the exposed areas which must remain White. In order to fix the print, it is treated for 7 to 10 minutes in a 7% aqueous solution of thiourea which also contains M/10 hydrochloric acid. The paper is then washed very rapidly in distilled water. This treatment causes the elimination of some dark spots which may remain after immersion in the acid bath. The print is then drained and dried under the conditions above described.

A blue positive image on a yellow background is obtained. The stability of the photographic reproduction is such that it can be kept for several days without alteration in a sunlit room or under a Wood lamp.

EXAMPLE 2 The procedure of Example 1 is followed except that the hydrochloric acid solution contains 20 to 30 mg. of quinone per liter. The photographic film is developed for 10 seconds in the leuco compound solution and then is dipped for 5 seconds in the solution of hydrochloric acid containing quinone.

EXAMPLE 3 The procedure of Example 1 is followed except that the film is developed for 30 seconds and then dipped for 15 seconds in an aqueous solution of 1 M hydrochloric acid containing 20 to 30 mg. of quinone per liter.

EXAMPLE 4 The general procedure of Example 1 is followed except that the developer is prepared as follows: A solution containing 2 grams of crystal violet in 1 liter of water is reduced with 10% aqueous freshly prepared sodium hydrosulfite solution until the color has just been discharged (about 10 cc. of sodium hydrosulfite are required). A small quantity of crystal violet is added until a permanent violet color is just formed.

The photographic paper is developed in the leuco compound solution for 20 seconds, immersed for 2 to 3 seconds in molar hydrochloric acid solution, then dried and fixed as in Example 1.

What is claimed is:

1. A process for producing a direct positive image in a latent image-exposed silver halide emulsion, without the use of an image-receiving layer, said process comprising the steps (1) developing a latent image in said silver halide emulsion up to a density less than 0.2 by contacting said silver halide with a neutral-to-alkaline aqueous solution of a leuco compound which is oxidizable to form a dark-colored dye that is soluble in said aqueous solution of a leuco compound, said leuco compound being absorbed unchanged in unexposed areas of said emul sion, and (2) contacting the developed emulsion with an aqueous solution of a hydrohalic acid containing a dissolved oxidizing agent whereby silver that was developed in exposed areas is dissolved leaving said exposed areas light in color, and leuco compound absorbed to unexposed areas of said developed emulsion is oxidized to a dark colored dye, forming said positive image.

2. A process for producing a direct positive image in a latent image-exposed silver halide emulsion, without the use of an image-receiving layer, said process comprising the steps (1) developing a latent image in said silver halide emulsion up to a density less than 0.2 by contacting said silver halide with a neutral-to-alkaline aqueous solution of a leuco compound which is oxidizable to form a dark-colored dye that is soluble in said aqueous solution of a leuco compound, said leuco compound being absorbed unchanged in unexposed areas of said emulsion, and (2) contacting the developed emulsion with an aqueous solution of a hydrohalic acid containing a dissolved oxidizing agent whereby silver that was developed in exposed areas is dissolved and leuco compound in said exposed areas remains unchanged, and leuco compound absorbed to unexposed areas of said developed emulsion is oxidized to a dark-colored dye, forming said positive image.

3. A process for producing a direct positive image in a latent image exposed silver halide emulsion without the use of an image-receiving layer, said process comprising the steps (1) developing a latent image in said silver halide emulsion up to a density less than 0.2 by contacting said silver halide with a neutral-to-alkaline aqueous solution of a leuco compound which is oxidizable to form a dark-colored dye that is soluble in said aqueous solution of a leuco compound, said leuco compound being absorbed unchanged in unexposed areas of said emulsion, and (2) contacting the developed emulsion with an aqueous solution of a hydrohalic acid containing a dissolved oxidizing agent whereby silver that was developed in exposed areas is dissolved leaving said exposed areas light in color, and leuco compound absorbed to unexposed areas of said developed emulsion is oxidized to a dark colored dye, forming said positive image, (3) heating the print to about 60-80 C., and (4) fixing said direct positive image.

4. The process of claim 1 wherein said positive image formed in step (2) is fixed in an aqueous acidic solution of thiourea.

5. The process of claim 1 wherein said oxidizing agent is atmospheric oxygen and said leuco compound is oxidizable to a dye by atmospheric oxygen.

6. The process of claim 1 wherein the leuco compound is the leuco derivative of methylene blue.

7. The process of claim 1 wherein the leuco compound is the leuco derivative of crystal violet.

8. The process of claim 6 wherein the said leuco derivative of methylene blue is dissolved in an aqueous medium having a pH of at least 7.

9. The process of claim 1 wherein hydrohalic acid is hydrochloric acid.

'10. The process of claim 3 wherein said oxidizing agent is atmospheric oxygen and said leuco compound is oxidizable to a dye by atmospheric oxygen.

11. The process of claim 3 wherein the leuco compound is the leuco derivative of methylene blue.

12. The process of claim 3 wherein the leuco compound is the leuco derivative of crystal violet.

13. The process of claim 3 wherein said leuco derivative of methylene blue is dissolved in an aqueous medium having a pH of at least 7.

14. The process of claim 3 wherein the hydrohalic acid is hydrochloric acid.

15. The process of claim 3 wherein said direct positive image is fixed in an aqueous acidic solution of thiourea.

References Cited UNITED STATES PATENTS 2,151,065 3/1939 Allison 96-54 2,266,456 12/1941 Wilder 9654 2,611,701 9/ 1952 Schoen 96-54 2,892,710 6/1959 Cohler et a1.

3,320,063 5/1967 Bloom 963 3,230,083 1/1966 Simon 9654 FOREIGN PATENTS 539,781 9/1941 Great Britain.

OTHER REFERENCES John and Field: Photographic Chem., 1963, p. 152.

J. TRAVIS BROWN, Primary Examiner US. 01. X.R. 

